(a) Field of the Invention
The present invention relates to a gain-coupled distributed feedback (DFB) semiconductor laser device and, more particularly, to a gain-coupled DFB semiconductor laser device having a single-longitudinal mode, lasing characteristic and capable of being fabricated with a higher yield rate (or higher percentage of the non-defected products to the total products).
(b) Description of the Related Art
A DFB laser device is known having a diffraction grating formed in the vicinity of an active area of the laser device, wherein only a laser component having a specific wavelength is fed-back to the resonator to allow the semiconductor laser device to have a lasing wavelength selectivity. The diffraction grating has a function for changing the real part and/or the imaginary part of the complex refractive index periodically with space in the resonator of the semiconductor laser device.
The DFB laser devices are categorized into two types including a refractive-index-coupled DFB laser which has a diffraction grating wherein only the real part of the refractive index is changed in the resonator, and a gain-coupled DFB laser (complex-coupled DFB laser) having a diffraction grating wherein both the real part and the imaginary part of the refractive index are changed in the resonator.
It is known that the conventional refractive-index-coupled DFB laser-device generally has a small difference in the threshold gain between a pair of modes sandwiching therebetween a Bragg wavelength. Thus, the refractive-index-coupled DFB laser device is liable to lasing in the pair of modes so that it is difficult to achieve the single-longitudinal mode lasing.
An effort is made to incorporate the gain-coupled structure in the refractive-index-coupled laser device, to enlarge the difference in the threshold gain between the pair of modes sandwiching therebetween the Bragg wavelength and to improve the yield rate with respect to the single-longitudinal mode lasing characteristic. The gain-coupled structure includes a diffraction grating in the vicinity of the laser active layer which gives the gain or loss of the stimulated emission and generates a feed-back at a certain wavelength thereby assisting the lasing in the single-longitudinal mode.
The term xe2x80x9cgain-coupled DFB laserxe2x80x9d as used herein means a laser structure wherein at least the imaginary part of the complex refractive index is changed periodically in space along the longitudinal direction of the elongate laser active layer.
The gain-coupled DFB laser device has advantages of excellent single-longitudinal mode selectivity and high endurance against external optical feedback. A gain-coupled DFB laser device having a wavelength shorter than 1 micrometer is generally important as a light source in a light measurement system, high-speed optical transmission system or optical storage device, whereas a gain-coupled DFB laser device having a wavelength longer than 1 micrometer is generally important as a light source in a long-distance optical transmission system.
In the DFB laser device; parameter xcexa is used for specifying the intensity of reflectance or feed-back ratio of the diffraction grating in the DFB laser device. The parameter K is defined by the coupling ratio of the field intensity of the backward wave to the field intensity of the forward wave in the laser. Refractive-index-coupling parameter xcexai and gain-coupling parameter xcexag are defined for the refractive-index-coupled DFB laser device and the gain-coupled DFB laser device, respectively.
The coupling parameter is determined by the optical confinement factor of the diffraction grating and by the difference between the real part and the imaginary part of the refractive index of the diffraction grating. In the gain-coupled DFB laser device, the gain-coupling coefficient xcexag is expressed by the following formula:                               κ          g                =                                            k              0              2                                      2              ⁢              β                                ⁢                      Γ            abs                    ⁢                                                    α                abs                            ⁢                              n                abs                                                    π              ⁢                              xe2x80x83                            ⁢                              mk                0                                              ⁢                      sin            ⁡                          (                              π                ⁢                                  xe2x80x83                                ⁢                mD                            )                                                          (        1        )            
wherein k0, xcex2, xcex1abs, nabs, m, D and xcex93abs are wave number of the free space, transmission factor in the Z-direction, absorption coefficient of the absorption layer, refractive index of the absorption layer, order of the diffraction grating, duty ratio of the diffraction grating and optical confinement factor of the part having a periodic absorption layer, respectively. In the above formula, xcex93abs is expressed as follows:   Γabs  =                              ∫          periodic                ⁢                                            E              ⁡                              (                x                )                                      ·                          E              ⁡                              (                x                )                                              *                      xe2x80x83                    ⁢                      ⅆ            x                                                ∫                      -            ∞                                +            ∞                          ⁢                                            E              ⁡                              (                x                )                                      ·                          E              ⁡                              (                x                )                                              *                      xe2x80x83                    ⁢                      ⅆ            x                                =          P      ·              d        abs            
wherein
x: direction normal to the substrate,
E(x): x-component of the electric field,
E(x)*: conjugate of E(x),
dabs: thickness of the optical absorption layer, and
∫periodic: integral over the periodic absorption layer
The gain-coupling coefficient xcexag is an important parameter, wherein a larger gain-coupling coefficient means that the laser device has an excellent single-longitudinal mode lasing characteristic. The laser characteristics such as threshold current largely depend on the gain-coupling coefficient xcexag, and the yield rate with respect to the single-longitudinal mode lasing characteristic also depends on the gain-coupling coefficient xcexag.
The types of diffraction grating used in the gain-coupled DFB laser device include two types: a gain-perturbative diffraction grating which periodically perturbs the gain of the active layer and an absorptive diffraction grating which effectively generates periodic perturbation of the gain by providing periodic optical absorption layers in the vicinity of the active layer.
The absorptive diffraction grating is extensively studied in consideration of the advantage of the fabrication feasibility thereof compared to the gain-perturbative diffraction grating. However, there is a problem in that the absorptive diffraction grating involves a poor reproducibility during the fabrication process, to thereby degrade the yield rate of the laser device with respect to the single-longitudinal mode lasing characteristic, and to have a poor reproducibility in the device characteristics.
Proposals have been made for solving the above problems. In an example of such a proposal, a gain-coupled DFB laser lasing at a wavelength of about 800 nm includes an absorptive diffraction grating having a duty ratio between 0.4 and 0.8 and a thickness of 6 to 30 nm for the absorption layer to improve the yield rate of the semiconductor laser device. It is reasoned that such a diffraction grating is well controlled during the fabrication.
The term xe2x80x9cduty ratioxe2x80x9d as used herein means the volumetric ratio of the periodic absorption layers to the whole layers in which the periodic absorption layers are disposed. If the cross section of the periodic absorption layer is rectangular, the duty ratio D (0xe2x89xa6Dxe2x89xa61) is obtained by the formula D=W/xcex9 wherein xcex9 is the length of one cycle of the diffraction grating and W is the width of the one cycle of the optical absorption layer.
The proposed technique has yet the following problems or tasks to be solved or finished.
First, although the duty ratio proposed therein resides between 0.4 and 0.8, it is suggested that the duty ratio should be as low as possible in view of the loss. Thus, it is important to find an appropriate range for the duty ratio and to raise the controllability and the reproducibility of the duty ratio within the appropriate range.
Second, the gain-coupling coefficient should be optimized for the semiconductor laser device having a longer resonator. This is because the resonator has become longer and longer to obtain a higher output power in the gain-coupled DFB laser device. In the above publication, the resonator discussed has a relatively small length such as about 250 micrometers, and thus if the resonator is longer than the recited length for achieving a higher output power, the above range for the duty ratio may not necessarily afford optimum device characteristics because the optimum value for the gain-coupling coefficient xcexag depends on the length of the resonator.
More specifically, it is not assured whether or not the optimum value for the gain-coupling coefficient in the semiconductor laser device having a relatively short resonator coincides with an optimum value for the gain-coupling coefficient for obtaining a higher yield rate in a semiconductor laser device having a single-longitudinal lasing mode. Thus, the optimum value for the gain-coupling coefficient should be determined for obtaining a higher yield rate with respect to the single-longitudinal mode lasing characteristic in a semiconductor laser device having a longer resonator.
Third, the design value for the gain-coupling coefficient should be determined to a concrete value. A lower gain-coupling coefficient xcexag does not afford effective gain-coupling and thus may degrade the yield rate with respect to the single-longitudinal mode lasing characteristic of the gain-coupled laser device down to a value comparable to the poor yield rate of the refractive-index-coupled laser device. In this case, the semiconductor laser device is liable to lasing at the pair of adjacent wavelengths sandwiching therebetween the stop band or Bragg, wavelength.
On the other hand, a larger gain-coupling coefficient xcexag raises the threshold gain difference between the pair of adjacent wavelengths sandwiching therebetween the stop band due to a larger influence by the gain-coupling coefficient. However, the yield rate of the semiconductor laser device with respect to the single-longitudinal mode lasing characteristic is also degraded because the other threshold gain difference is lowered between adjacent two lasing wavelengths disposed on one side of the stop band.
In the publication, the upper limit is determined solely by the absorption loss. However, the upper limit of the optimum gain-coupling coefficient is considered to be lower than the value discussed in the above publication.
In view of the above, it is an object of the present invention to provide a gain-coupled DFB semiconductor laser device which has a relatively low duty ratio and a diffraction grating having a gain-coupling coefficient that is optimum for the single-longitudinal mode lasing characteristic and a higher yield rate of the semiconductor laser device.
The present invention provides a gain-coupled DFB semiconductor laser device including a substrate, at least one elongate active layer for stimulated laser emission, and a diffraction grating overlying the laser active layer to feed-back the laser emission thereby allowing the laser active layer to lase, the diffraction grating having a structure wherein gain or loss of the laser emission changes periodically along an extending direction of the elongate active layer, a gain coupling coefficient xcexag and a length Lg of the diffraction grating satisfying the following relationship:
0.5xe2x89xa6|xcexag|xc3x97Lgxe2x89xa61.1.
In accordance with the gain-coupled DFB semiconductor laser device of the present invention, the specific range of the product |xcexag|xc3x97Lg affords a higher yield rate of the DFB semiconductor device with respect to the single longitudinal mode lasing characteristic, thereby reducing the total cost for the gain-coupled DFB semiconductor laser device.